Relativity of simultaneity objections

[DrGreg]

...because he is actually moving despite the fact that he doesn't know it.

No is isn't "actually moving". There's no such thing as "actually moving". The only sort of motion is one thing relative to something else. He's fully entitled to consider himself at "actually at rest". The laws of physics don't care whether his box is moving relative to something else or not. The person always measures the speed of light to be c, no matter how fast the box is moving relative to something else (as long as the motion is uniform, i.e. not accelerating).

It sounds difficult to believe, but it's the assumption on which all of relativity is based. So if you find something else that seems to contradict this assumption, it must be the "something else" that is wrong, if you accept relativity.

 

[Nugatory]

no i don't think that this is bugging me. I think the computer simulation example was pretty good. So in the computer simulation of this we have a box which moves in the screen at 10pixel/second to the right. inside the box there is a light emitter ( which also moves at 10 pixel/second ), at some point the emitter emits light in both directions which travels at 300 pixel / second in EACH direction. So the box itself doesn't "know" if it's moving or not, BUT the light beams will reach the front of the box later than it will reach the rear one

Take a look at my post #37 and the comment about water waves. You're using the surface of the screen to say that the light only travels at speed c relative to the screen surface, not everything else. That's a good analogy for water waves moving through the water at a constant speed relative to the water - but it doesn't work for light in a vacuum, where there's nothing but you and the light and no computer screen or water surface or other background to check your speed against.

 

[Nugatory]

someone "outside" of the universe could see their absolute movements (no matter how complicated they would be )

Maybe, but that's a question for the philosophers who can talk about unmeasurable unprovable things such as what "outside the universe" might mean. Physics is the science of what we can, at least in principle, measure and observe.

With tongue slightly in cheek, I'm going to suggest that your position can be paraphrased as "If I believe that if there were such a thing as 'outside the universe', and if there were an observer there, and if he the saw the world in a particular way, then I would find it very hard to sensibly explain all the observations that we living in the universe have made". That's true enough, but the easy way out of that dilemma is to not make those assumptions about things "outside the universe".

That's essentially what Einstein's first postulate suggests that we do, although the notion is far older.

One caution: Because physics is a science of what we can in principle measure, threads that wander too far off into philosophical questions about things that cannot in principle be measured tend to get locked on the grounds that they're no longer about physics. (They also tend to become intellectually sterile well before that point).

 

[mirrormirror]

Take a look at my post #37 and the comment about water waves. You're using the surface of the screen to say that the light only travels at speed c relative to the screen surface, not everything else. That's a good analogy for water waves moving through the water at a constant speed relative to the water - but it doesn't work for light in a vacuum, where there's nothing but you and the light and no computer screen or water surface or other background to check your speed against.

Yes i understand what you mean, that the computer screen acts like sort of an "ether" but i think the analogy is not valid. The simulation doesn't happen in the screen, it happens in computer memory and in the cpu ( so it's just mathematics ). The screen is just the visual representation medium of the universe. So write off that 300 pixels/second speed. Let's say that light in the simulation moves at 300.000.000 m /second ( just like in real life ) and that for practical reasons we represent 1.000.000 to 1 pixel in the screen or else we would need a huge screen!

 

[Nugatory]

I understand what you mean, that the computer screen acts like sort of an "ether" but i think the analogy is not valid. The simulation doesn't happen in the screen, it happens in computer memory and in the cpu so it's just mathematics)

Yes, it's just mathematics, but it would be mathematics that doesn't accurately simulate the behavior of the real world. If I were going to make it accurate, I'd have to add another option to the menu bar: "Show how it all looks for an observer represented by a pixel moving at speed X relative to the top left corner of the screen" and there would be absolutely nothing special about the view that I get when I choose X=0.

There are some pretty decent relativistically accurate simulations out there.
Here's one that you might try: http://gamelab.mit.edu/games/a-slower-speed-of-light/

 

[mirrormirror]

i think that what i fail to grasp is WHY ( that is, for what practical reasons ) the light beams will reach the MOVING box doors at the same time, like all of you said. The way i think of it, exactly due to the fact that c is constant and invariant, it will take it more time to reach the front clock than the rear one, because the front one is moving away from it and the back moving towards it. The observer inside the box doesn't know this, but he will notice it when he he sees the front clock stopped at 12:00:11 and the back clock stopped at 12:00:10.

Janus said ( in post 27 ), experiments showed they both will stop at 12:00:10, it's that I can't see how that can happen practically.

Anyway i will sleep over it and think it again. Thank you all for your help and your CONSTANT AND INVARIANT patience :-)

 

[WannabeNewton]

There is no universal coordinate time in SR as there is in Newtonian mechanics. Maybe this is what is confusing you? It is an alien concept at first for sure.

 

[darkhorror]

There is no absolute frame of reference. This is a fact. the speed of light is constant for all inertial frames of reference, this is also a fact. These are tested and observed.

If you would like to learn about relativity and how the universe works you need to get rid of your preconceived notions of how you think the universe works.

 

[darkhorror]

Think of it like this let's say I am standing on Earth and there is a spaceship moving away from me at .9c. In Earth's frame of reference light is moving .1c away from the ship in the same direction of it's motion. While in the ships frame of reference that light is moving away from it at c, and in the ships frame of reference it's speed is 0, and the Earth is moving away from it at .9c. Also in the ships frame of reference light is moving away from the Earth at .1c in the direction of motion. While in Earth's frame of reference that light is moving away from the Earth at c.

 

[phinds]

... doesn't mean that there is no TOTAL DEFINITE ABSOLUTE movement of each of them in the universe (though i think that according to my rationale they can find it out). Someone "outside" of the universe could see their absolute movements

This is nonsense. You can repeat it to yourself as often as you like --- make it your mantra in fact --- and it will still be nonsense.

 

[Janus]

i think that what i fail to grasp is WHY ( that is, for what practical reasons ) the light beams will reach the MOVING box doors at the same time, like all of you said. The way i think of it, exactly due to the fact that c is constant and invariant, it will take it more time to reach the front clock than the rear one, because the front one is moving away from it and the back moving towards it. The observer inside the box doesn't know this, but he will notice it when he he sees the front clock stopped at 12:00:11 and the back clock stopped at 12:00:10.

Your not getting what invariant means.

Here's an example. You have two clocks, At first we look them from the perspective of someone at rest with respect to the clocks. A flash of light is emitted from a point halfway between them. It expands outward at c equally in all directions, strikes the clocks and starts them running. They remain in sync

Now we consider the same clocks and the same light flash from the perspective from someone to which the clocks are moving left to right. Again, the flash is emitted from midway between the two clocks and again expands outward at c equally in all directions. This is what 'invariant' means; All inertial frames measure light as moving at c with respect to themselves.

As a result, in this frame, the light hits the left clock before hitting the right clock.

The left clock starts first and then the right clock. The clocks are not in sync. Let me repeat these are the same clocks and the same light flash as in the first example, just considered from a different inertial frame.

As to why this is the case, it is because this is how time and space relate to each other.

I'll try to use an analogy. Absolute time and space, like you are proposing are like the directions North-South and East-West. If you ask someone how far North and far West Chicago is from Miami, everyone gives the same answer.

However, what we have found that time and space aren't like North-South and East-West. They are more like Left-right and forward-backward. If you ask two people how far to the left and how far forward Chicago is from Miami, they will give you different answers if they are facing in different directions relative to each other. And you can't say which one is "really" correct, because the whole concept of Left and Right are dependent of the person.

In the same way, time and space are frame dependent. One frame can say that two events are simultaneous and another will say that they are not in the same way that one person facing one direction will say that two objects are directly to the right and left of each other, while another person facing in a different direction will say that they are not, and that one object is forward of the other.

 

[Mentz114]

i think that what i fail to grasp is WHY ( that is, for what practical reasons ) the light beams will reach the MOVING box doors at the same time, like all of you said. The way i think of it, exactly due to the fact that c is constant and invariant, it will take it more time to reach the front clock than the rear one, because the front one is moving away from it and the back moving towards it. The observer inside the box doesn't know this, but he will notice it when he he sees the front clock stopped at 12:00:11 and the back clock stopped at 12:00:10.

This is absolutist troll rubbish. You've been told a dozen times that the clocks will show the same reading.

 

[Dale]

i think that what i fail to grasp is WHY ( that is, for what practical reasons ) the light beams will reach the MOVING box doors at the same time, like all of you said.

Special relativity is based on two postulates. The second is the invariance of c, but the first is the principle of relativity. The principle of relativity means that the laws of physics do not change depending on which inertial frame you are using. This means that the laws of physics inside a stationary box are exactly the same as the laws of physics inside a moving box.

So, use some procedure to synchronize the clocks and set up the light beams. Then, it doesn't matter if you did that procedure in a moving or stationary box, the result will be the same.

 

[ghwellsjr]

Imagine if we made a computer simulation of this: the simulation KNOWS that the box is moving (because it's a parameter you set, it's in the initial data), thus the light beam will reach it later ( in the front ) than in the back.

So for example if in the computer simulation light moves at an invariant speed of 300pixel / second and the box at 10 pixel / second, the light beam will reach the front door later than the back.

Good idea. I made a series of animations to illustrate how the early scientists explained the Michelson-Morley Experiment (or could have) and how Einstein came up with an alternate explanation.

Take a look, study it, and see if it helps you.

 

[Chestermiller]

sorry, but why will he notice that the clocks did not stop at the same time? The clocks are AT the doors. One clock in the back door one in the front. Once they get hit by the strikes they stop.

If I told you that this is what would be observed experimentally, what would you say? If this same experiment were repeated over and over again, the same result would be obtained. In the frame of reference of the guy on the train, the clock at the front of the train would be found to be hit first, and then the clock at the back of the train.

All the clocks on the ground are synchronized with one another, and all the clocks on the train are synchronized with one another. But, unfortunately, the people on the ground would observe that, according to their reckoning, the clocks on the train are out of sync, and the people on the train would observe that, according to their reckoning, the clocks on the ground are out of sync. This is why two strikes can occur at the same time as reckoned by the people on the ground, while, the people on the train would reckon that the two strikes did not occur at the same time. Who is correct? They both are, according to their respective sets of synchronized clocks.

Another way of look at this is that the people on the ground are able to see partially into the past and partially into the future of the guy at the center of the train.